A Mach-Zehnder directional coupler, such as that shown in FIG. 1 of this application, comprises two identical single mode guide elements, A and B, and comprises two couplers. The first coupler C1, on the left, has a length .LAMBDA./4, where .LAMBDA. is the coupling period, and produces the transfer effect towards the guide element B of half the optical power which has entered the guide A. In guide B, at the output of coupler C1, the light wave leads in phase of .pi./2. At the other end, the second coupler C2, has an identical length, and is capable of providing either the transfer to the guide B of the second half of the light if the phase at the input of the coupler C2 is still leading in phase .pi./2 at the input of the coupler C2, coupler C2 returns the light from guide B to guide A if the phase lags by .pi./2. The function of the switching unit from one channel to the other is ensured by means of an electro-optical phase modulator M.
By way of information, with regard to Mach-Zehnder couplers, reference may be made to the technical study "Fibre-optic systems" by P. Halley, published by Eyrolles in 1985, page 143.
In the technical study "Transmissions by fibre optics", by Y. Suematsu et al., published by Masson in 1984, pages 110 and 111, there is likewise a disclosure and description of an optical switching device consisting of two phase modulators, operating respectively on each branch of a Mach-Zehnder coupler.
An initial object of the present invention includes providing, as the electro-optical modulator in a Mach-Zehnder coupler, a liquid crystal ferroelectric electro-optical coupler which engenders bi-refractivity modulation by the rotation of the optical axis, and which allows for phase modulation, which is adapted to the function principles of a Mach-Zehnder coupler.
By way of further information, both the principle and the obtaining of an index modulation, or of a binary or analog phase in a ferroelectric liquid crystal, have been described in the technical article "Programmable phase-only optical device based on ferroelectric liquid crystal SLM" by S. E. Broomfield et al., which appeared in the British publication Electronics Letters Jan. 2, 1982, Vol. 28, pp. 26-23, and in the technical article "High speed analog refractive index modulator that uses a chiral smectic liquid crystal", by A. Sneh et al., which appeared in the American Journal Optics Letters of Feb. 15, 1994, Vol. 19. No. 4, pp. 305-307.
In relation to other electro-optical modulators, liquid crystal modulators are interesting solutions due to the fact that substantial electro-optical effects are obtained under a weak electrical field, and at low cost. With smectic liquid crystals, it is possible to achieve practical switching times of a few tens of microseconds. Among the smectic liquid crystals which allow for binary phase modulation and which can be controlled almost perfectly, C* smectics are the best suited for the application envisaged according to the invention, for allowing for modulation (0, .pi.). A second quality relates to the insensitivity to polarisation. More precisely, the electro-optical modulator must be insensitive simultaneously to the state and to the orientation of the polarisation, to the extent that it is no longer possible to control the state of input polarisation of the Mach-Zehnder coupler, or at least to control its orientation.
By way of still further information, different solutions have already been described and proposed to overcome this disadvantage, in this context. For example, reference is made to the document WO-A-94 25893 and to the technical article "Polarisation-insensitive operation of ferroelectric liquid crystal devices", by S. T. Warr et al., which appeared in the British journal Electronics Letters of Mar. 6, 1995, Vol. 31, No. 9, pp. 714-716.
In addition to this interest has been shown in the use of a liquid crystal at a broad tile angle .theta., close on 45.degree., and operating in half-wave to provide binary phase converters, with a minimum of losses and insensitive to polarisation.
This type of component appears to be extremely well-suited for providing an electro-optical phase modulator which can be used in a directional Mach-Zehnder coupler.
According to the invention, it is proposed to make use of an SSFLC (surface stabilized ferroelectric liquid crystal) cell in an electro-optical phase modulator, such as a liquid crystal, the particular feature of which is the provision of a wide tilt angle .theta. which should be as close as possible to 45.degree..
FIG. 2 shows a schematic representation of the known principles of operation of a traditional SSFLC cell. The cell CL has perpendicular layers at the walls P1 and P2, constituted by two large rectangular surfaces. The thickness of the cell CL is the distance between the two walls P1 and P2, and is selected to be equal to .LAMBDA./2; i.e. this is a half-wave cell. The cell CL and the transparent walls P1 and P2 are assumed to transmit a light bundle Z, the incident wave level of which is symbolically represented in front of the cell CL by its electromagnetic field vectors E and H. The transparent walls P1 and P2 are electrical conductors and can be submitted to a difference in electrical potential +E, indicated by an arrow tail or -E, indicated by an arrow head.
With a cell CL in which the tilt angle .theta. is close to 45.degree., for a difference in potential of +E, which corresponds to the state E1, representing in schematic form the orientation of the molecules, and for a difference in potential -E, which corresponds to the status E2, representing schematically the switched orientation of the molecules, it can be seen that, between the state E1 and the state E2 the orientation of the molecules has turned through .pi./2. On the right in FIG. 2, the state E1 is symbolically represented by an ellipse, the main axis of which carriers the vector n.sub.e (extraordinary index), and the lesser axis of which bears the vector n.sub.o (ordinary index).
By way of example given that .DELTA.n=0.12 for .LAMBDA.=546 mm, and that .DELTA.n=0.1 for .LAMBDA.1550 nm, a thickness of the cell can be derived in the order of 7 .mu.m in order to obtain a half-wave. The response time is 126 .mu.s for a voltage of 10 V/.mu.m at a temperature of 25.degree. C., and the spontaneous polarisation is 72 nC/cm2. It may be noted that, for a voltage of 20 V/.mu.m, the response time would be 60 .mu.s.
In the principle diagram in FIG. 2, it can be seen that the optical axis of the liquid crystal rotates through 90.degree. if .theta. is equal to 45.degree., by changing the sign of the electrical field applied, E. When the thickness of the liquid crystal is a half-wave, a phase variation is obtained which is equal to .pi. between the two individual axes, whatever the state of polarisation might be.
Assume a state of rectilinear polarisation of any orientation at all. A transition to a state of any elliptical polarisation will exert no change in the principle of phase modulation in the context of rectilinear polarisation when the anisotropic phase displacement .phi. is equal to .pi. and .theta. is equal to 45.degree..
FIG. 3 which represents a diagram showing the orientation of the two neutral axes of an SSFLC cell used in the embodiments of the invention. Assume of an incident rectilinear polarisation Pi, oriented in accordance with Pv. In determining the references for the horizontal rectilinear polarisation Ph and vertical polarisation Pv, the Kones matrix of an SSPLC liquid crystal is, in general: ##EQU1## where .theta. is the tile angle, s is a function of the position of the neutral axes in relation to the vertical polarisation Pv and .phi.=2.pi.e/.LAMBDA.(n.sub.s +n.sub.o) is the anisotropic phase-displacement where. The constant phase displacement .phi. is=.pi.e/.LAMBDA.(n.sub..phi. +n.sub.o) has been ignored.
In the situation in which .phi.=.pi. and if the input polarisation Pi is vertical, the output polarisation Ps is: EQU Ps=-i(cos 2s.theta.Pv+sin 2s.theta. Ps)
If .theta.=45.degree. is chosen, in the situation in which s=2.sup.n, corresponding to the state E1 of the liquid crystal: EQU Ps1=-i(cos 4.alpha..theta. Pv+sin 4.alpha..theta. Ps)
and in the situation in which s=2(-1), corresponding to the second state of the liquid crystal, designated E2; EQU Ps2=+i(cos 4.alpha..theta. Pv+sin 4.alpha..theta. Ps)
The relationships indicated above illustrate the method for obtaining two identical states of polarisation at the output, but phase-displaced between one another by .pi..
Such conditions are fulfilled, for example, by using the material CS-2005 from Chisso for the SSFLC cells, and by adjusting the thickness of the cell in order to obtain a half-wave at the operational wavelength of the coupler.